Abstract
Cardiac transcription factors (TFs) directly reprogram fibroblasts into induced cardiomyocytes (iCMs), where MEF2C acts as a pioneer factor with GATA4 and TBX5 (GT). However, the generation of functional and mature iCMs is inefficient, and the molecular mechanisms underlying this process remain largely unknown. Here, we found that the overexpression of transcriptionally activated MEF2C via fusion of the powerful MYOD transactivation domain combined with GT increased the generation of beating iCMs by 30-fold. Activated MEF2C with GT generated iCMs that were transcriptionally, structurally, and functionally more mature than those generated by native MEF2C with GT. Mechanistically, activated MEF2C recruited p300 and multiple cardiogenic TFs to cardiac loci to induce chromatin remodeling. In contrast, p300 inhibition suppressed cardiac gene expression, inhibited iCM maturation, and decreased the beating iCM numbers. Splicing isoforms of MEF2C with similar transcriptional activities did not promote functional iCM generation. Thus, MEF2C/p300-mediated epigenetic remodeling promotes iCM maturation.